1. Technical Field of the Invention
The present invention relates to a piezoelectric resonator which is adapted to vibrate in a longitudinal vibration mode, and electronic components including such a piezoelectric resonator.
2. Description of the Related Art
Piezoelectric resonators utilizing mechanical resonance of a piezoelectric member include, for example, a piezoelectric resonator in which electrodes are provided on both surfaces of a piezoelectric plate which is polarized along a thickness direction thereof. In this type of piezoelectric resonator, an AC electric field is applied between the electrodes to excite a longitudinal vibration or a lateral vibration. However, this type of piezoelectric resonator utilizes an unstiffened mode in which the direction of an electric field and the direction of polarization are different from the direction of vibration. As a result, this type of piezoelectric resonator has a small electromechanical coupling coefficient and a relatively small difference .DELTA.F between the resonant frequency and the anti-resonant frequency.
The use of such an unstiffened-mode piezoelectric resonator in an oscillator or a filter disadvantageously results in a small bandwidth and a low degree of flexibility in designing the electronic component characteristics. To overcome such disadvantages, there has been proposed a piezoelectric resonator having a laminated structure, such as the one shown in FIG. 15, which is a stiffened-mode piezoelectric resonator having a large electromechanical coupling coefficient and a relatively large difference .DELTA.F between the resonant frequency and the anti-resonant frequency.
FIG. 15 is an illustration showing an example of a piezoelectric resonator utilizing the stiffened mode. A piezoelectric resonator 1 includes a rectangular-shaped base member 2. The base member 2 is formed by alternately laminating a plurality of piezoelectric layers 3 and a plurality of internal electrodes 4. Two piezoelectric layers 3 that are adjacent to each other with one internal electrode 4 therebetween are polarized in opposite directions in the longitudinal direction of the base member 2. A groove 5 is formed on one lateral surface of the base member 2, and the internal electrodes 4 are exposed at every other layer on both lateral surfaces of the groove 5. Further, on both sides of the grooves 5, external electrodes 6a and 6b are provided on the lateral surface of the base member 2. Accordingly, the adjacent internal electrodes 4 are connected to the different external electrodes 6a and 6b, respectively.
In the piezoelectric resonator 1, a signal is input into the external electrodes 6a and 6b, thereby applying an AC electric field between the adjacent internal electrodes 4. Since the adjacent piezoelectric layers 3 are polarized in opposite directions, upon applying an AC electric field to the layer 3, the individual piezoelectric layers 3 expand or contract in the longitudinal direction of the base member 2. As a result, a longitudinal fundamental vibration is generated in the entire piezoelectric resonator 1. In this manner, the piezoelectric resonator 1, which utilizes the stiffened mode in which the direction of an electric field and the direction of polarization coincide with the direction of vibration, can increase the electromechanical coupling coefficient and .DELTA.F in comparison with an unstiffened-mode piezoelectric resonator. Thus, the use of a stiffened-mode piezoelectric resonator in an oscillator or a filter makes it possible to increase the bandwidth and increase the degree of flexibility in designing the component characteristics.
In the above type of piezoelectric resonator, however, a capacitance is generated between the unexposed portion of each internal electrode and the external electrode which is not connected to the unexposed internal electrode. This capacitance is unfavorably connected in parallel to the capacitance between the internal electrodes, which causes vibration, thereby lowering the capacitance ratio of the resonator and further decreasing the difference .DELTA.F between the resonant frequency and the anti-resonant frequency. This results in a small bandwidth when the piezoelectric resonator is used in an oscillator or a filter.